Brownout detector system and method

ABSTRACT

A circuit for detecting a brownout condition may include a temperature compensating circuit that provides a temperature compensated brownout reference voltage and an input signal to compare with the brownout reference voltage. Additionally, the detection circuit may include a comparator to generate a brownout indicator if the input signal crosses the brownout reference voltage. In certain implementations, the temperature compensating circuit has two branches connected in parallel. A first branch provides the reference voltage substantially independent of temperature, and a second branch provides the input signal, which is an indication of a supply voltage. The first branch is coupled to a first input of the comparator, and the second branch is coupled to a second input of the comparator. Additionally, the temperature compensating circuit may include a third branch that has components to adjust the reference voltage.

FIELD

The present invention relates generally to electrical circuits.

BACKGROUND

A brownout detector is a device that can be used, for example, in acomputing device, to detect when supplied power to the device fallsbelow a predetermined threshold level. Some conventional brownoutdetectors use a comparator to compare a power supply voltage with abandgap reference voltage provided by a bandgap circuit. If the dividedpower supply voltage drops below the bandgap reference voltage, thecomparator output signal can be used to indicate a brownout. The bandgapcircuit can provide a temperature compensated voltage threshold,referred to as the bandgap reference voltage. The bandgap circuit mayrequire several components including diodes, resistors, and an amplifierto generate a voltage threshold independent of temperature changes.

SUMMARY

A circuit for detecting a brownout condition may include a temperaturecompensating circuit that provides a temperature compensated brownoutreference voltage and an input signal to compare with the brownoutreference voltage. Additionally, the detection circuit may include acomparator to generate a brownout indicator if the input signal crossesthe brownout reference voltage. In certain implementations, thetemperature compensating circuit has two branches connected in parallel.A first branch provides the reference voltage substantially independentof temperature, and a second branch provides the input signal, which isan indication of a supply voltage. The first branch is coupled to afirst input of the comparator, and the second branch is coupled to asecond input of the comparator. Additionally, the temperaturecompensating circuit may include a third branch that has components toadjust the reference voltage.

In other implementations, a method for brownout detection is described.The method includes setting a brownout threshold voltage using a firstbranch of a temperature compensating circuit. The brownout thresholdvoltage is substantially independent of temperature changes. The methodincludes providing a second voltage using a second branch of thetemperature compensating circuit in parallel with the first branch. Thesecond voltage indicates a power supply voltage. The method alsoincludes generating a signal from a comparator when the second voltagecrosses the brownout threshold voltage. The brownout threshold voltageis coupled to a first input of the comparator and the second voltage iscoupled to a second input of the comparator.

In yet other implementations, a circuit is described that includes adetection component to detect a crossing of a reference voltage by aninput voltage, and a temperature compensating circuit to generate thereference voltage. The temperature compensating circuit includes twocircuit branches connected in parallel and separately coupled to thedetection component, a first circuit branch to generate the referencevoltage being substantially independent of temperature, and a secondcircuit branch to generate the input voltage.

In other implementations, a method for brownout detection is described.The method includes setting a brownout threshold voltage, maintainingthe brownout threshold voltage substantially independent of temperaturechanges including compensating temperature changes in parallel circuitelements to substantially maintain the brownout threshold voltage at asubstantially constant level, evaluating a second voltage indicative ofa power supply voltage, and generating a signal when the second voltagecrosses the brownout threshold voltage.

The systems and techniques described here may provide one or more of thefollowing advantages. First, a temperature compensated reference voltagemay be provided without the use of a bandgap circuit, which may reducethe number of components in the system. Second, accuracy of a brownoutdetector may be maintained with less expensive and complex components.Third, a system may provide a temperature compensated reference signaland require less power consumption. Fourth, a system may includecomponents that facilitate an adjustable brownout threshold.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the embodiments will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an exemplary system for detecting brownoutsthat includes a temperature compensation circuit.

FIG. 2 is a flow chart of an exemplary method for detecting brownoutsusing the system of FIG. 1.

FIG. 3 is a schematic of an exemplary temperature compensating brownoutcircuit.

FIG. 4 is a schematic of an exemplary temperature compensating brown outcircuit having a variable brownout voltage reference.

FIG. 5 is a schematic diagram of a general computer system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A brownout detector can be used to monitor a power supply voltage in acomputing device. The brownout detector can provide a signal, in theform of a voltage level, when the power supply voltage drops below apredefined voltage threshold. In some implementations, a circuit thatincludes solely resistors and diodes can generate a temperaturecompensating voltage threshold. A comparator can be used to compare thevoltage threshold to a power supply voltage or a voltage derived fromthe power supply voltage. If the power supply voltage drops below thevoltage threshold, the comparator can generate an output signal toindicate a brownout. In some embodiments, the computing device uses thebrownout indicator to initiate a series of lower power routines, such asperforming an orderly shutdown of the device or powering downnon-essential components to preserve operating power.

FIG. 1 is a block diagram of an exemplary system 100 for detectingbrownouts. The system 100 includes a power supply 102 and a device. Inthe example shown, the device is a computing device 104, though otherdevices are possible. By way of example reference will be made to animplementation that is included in a computing device. Those of ordinaryskill in the art will recognize that the circuits and methods disclosedcan be used with other devices.

The power supply 102 can be used to supply power to the computing devicefor its operation. The power supply 102 can supply a signal having avoltage level, U_(P). The computing device 104 can include a brownoutdetector 106 and a processing unit 108. The computing device 104 canutilize the processing unit 108 to control, monitor, and executeoperations. The brownout detector 106 can include a temperaturecompensating circuit 110 and a comparator 112. The temperaturecompensating circuit 110 and the processing unit 108 can receive theoutput signal of the power supply 102 as an input signal with thevoltage level, U_(P).

The temperature compensating circuit 110 can provide a temperaturecompensated voltage reference to an input 114 of the comparator 112. Insome implementations, temperature compensation can be achieved in thecircuit as temperature increases by the summation of an increasingvoltage level and a decreasing voltage level. The net change in voltageover temperature can be approximately zero. For example, voltage acrossdiodes included in the circuit may decrease at a rate of approximately 2mV/K (process dependent) as the temperature increases:V _(D) =k ₁·(T−T ₀)+V _(D0)

A bandgap reference circuit exploits this by making a current that isproportional with absolute temperature (PTAT):

$I_{P\; T\; A\; T} = {\frac{1}{R_{2}} \cdot \left( {\frac{k_{B} \cdot T}{q} \cdot {\ln(A)}} \right)}$When this current is run through a resistor and a diode in series thefollowing equation is obtained:V _(BGAP) =V _(D) +R ₃ ·I _(PTAT)Here the first term decreases with increasing temperature, while thesecond term increases with temperature. These voltages may be designedto cancel out and give a constant voltage, which has the equation:

$U_{BOR} = {{k_{1} \cdot \left( {T - T_{0}} \right)} + V_{D0} + {\frac{R_{3}}{R_{2}} \cdot \left( {\frac{k_{B} \cdot T}{q} \cdot {\ln(A)}} \right)}}$

The temperature compensated voltage reference is referred to here as abrownout reference level, U_(BOR). The brownout reference level can beused to detect a decrease or loss of supplied power. This detection canoccur when the supplied power (e.g., from power supply 102) crosses thebrownout threshold level. The temperature compensating circuit 110 canalso provide a signal (U_(P′)), which indicates the current level of thepower supply 102. In some implementations, the signal (U_(P′)) is adivided power supply voltage generated by a voltage divider intemperature compensating circuit 110 and is provided to an input 116 ofcomparator 112. The comparator 112 can determine when the power supplysignal U_(P′) (e.g., the divided power supply voltage level) falls belowor exceeds the temperature compensated brownout reference U_(BOR). Whenthis occurs, the comparator 112 can cause a comparator output 118 totransition from a first voltage level to a second voltage level. Forexample, the first voltage level may be substantially 2.5 volts and thesecond voltage level may be substantially zero volts. The signal output118 of the comparator 112 is referred to here as a brownout indicatorsignal, U_(BO).

In some implementations, the computing device 104 uses the brownoutindicator signal to control its operation under lower power supplyconditions. In other implementations, the computing device 104 uses thebrownout indicator signal to determine when the computing device isbeing powered up. In this case, the output 118 may transition fromsubstantially zero to substantially 2.5 volts. The computer device 104can use this output signal U_(BO) to initiate power up events, such asresetting the processing unit 108 to a default state.

FIG. 2 is a flow chart of an exemplary method 200 for detectingbrownouts using the system of FIG. 1. The method 200 begins at step 202when the computing device 104, as described with reference to FIG. 1,receives a power supply signal from, for example, the power supply 102.The temperature compensating circuit 110 receives the power supplysignal as an input.

Under some conditions, a system temperature change may be detected, asshown in an optional step 204. If there has been a system temperaturechange, in step 206, the temperature compensating circuit will performtemperature compensation. For example, the circuit may increase intemperature as it functions. Current flowing through the circuit'scomponents may experience resistance, which creates heat. As some of thecomponents of the temperature compensating circuit 110 become hotter,these components may contribute to an increase in the output voltage ofthe circuit. Simultaneously, as other components of the temperaturecompensating circuit 110 become hotter, they may contribute to adecrease in the output voltage of the temperature compensating circuit110. Because of the opposite, or compensating, changes in voltages, thenet output voltage may remain constant regardless of the temperaturechanges within the circuit. More details of this compensation aredescribed in association with FIG. 3.

After the step 206 or if a system temperature change is not detected instep 204, the method 200 proceeds to step 208. In step 208, it isdetermined if the power supply voltage (e.g., the divided power supplyvoltage, which is the input 116 of comparator 112) crosses the brownoutreference voltage (which is the input 114 of comparator 112). If thisoccurs, in step 210, a brownout indicator signal is generated. Forexample, the brownout indicator signal can be the signal output 118 ofcomparator 112. The processing unit 108 can receive the brownout signalindicator from the signal output 118 of the comparator 112.

Optionally, in step 212, the computing device 104 may take action basedupon the brownout indicator signal. For example, the computing devicemay power down non-essential components so that functionality can bemaintained by the processing unit 108 at the lower power supply voltagelevel. The method 200 then ends.

If, in step 208, it is determined that the power supply voltage did notcross the brownout reference voltage, the method 200 proceeds to step202 where it continues to receive the power supply signal from the powersupply.

FIG. 3 is a schematic of an exemplary temperature compensating brownoutcircuit 300. The temperature compensating circuit 110 of the brownoutcircuit 300 includes resistors R₁ 302, R₂ 304, and R₃ 306, and diodes D₁308 and D₂ 310. Outputs of the temperature compensating circuit 110 arecoupled to inputs of a comparator 312.

In a first branch of the temperature compensating circuit 110, terminal314 of resistor R₁ 302 is coupled to a power supply voltage V_(CC) 316.Terminal 318 of resistor R₁ 302 is coupled to terminal 320 of resistorR₂ 304 at node 322. Terminal 324 of resistor R₂ 304 is coupled to ananode 326 of diode D₁ 308. A cathode 328 of diode D₁ 308 is connected toa power supply ground 330.

In a second branch of the temperature compensating circuit 110, terminal332 of resistor R₃ 306 is also coupled to the power supply voltageV_(CC) 316. Terminal 334 of resistor R₃ 306 is coupled to an anode 336of diode D₂ 310 at node 338. A cathode 340 of diode D₂ 310 is connectedto the power supply ground 330. In this way, the first and secondbranches of the temperature compensated circuit are connected inparallel because they share a connection to the power supply voltageV_(CC) 316 at one end and the ground point 330 at the other end.

Node 322 is coupled to a negative or inverting input 342 of comparator312. The negative input 342 of comparator 312 is referred to as U_(P′)described in reference to FIG. 1. The negative input 342 is anindication of the current power supply voltage. Node 338 is coupled to apositive or non-inverting input 344 of comparator 312.

The positive input 344 of comparator 312 is referred to as a brownoutreference level, U_(BOR), described in reference to FIG. 1. The brownoutreference level sets a threshold, which when crossed indicates abrownout condition. An output 350 of the comparator 312 is the brownoutindicator signal, U_(BO), described in reference to FIG. 1. Thecomparator 312 is powered by the power supply voltage V_(CC) 316 atterminal 346 and is grounded by the power supply ground 330 at terminal348.

Current through a diode as a function of temperature and diode voltagecan be characterized by the following equation:

${I\left( {V_{D},T} \right)} = {I_{S} \cdot^{\frac{V_{D}}{k_{B} \cdot \frac{T}{q}}}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Temperature, T

Diode voltage, V_(D)

Saturation current (process dependent), I_(S)

Voltage across a diode may not change significantly as a function of thecurrent through it. Therefore, the voltage across a diode can beapproximated by the following equation:V _(D)(T)=k ₁·(T−T ₀)+v _(D0)

where

Temperature, T

Room temperature, T₀=300 K

Diode voltage, V_(D)

Diode voltage at room temperature (which is process dependent),V_(D0)=0.65 V

The equation above, therefore, may be used to characterize the voltageat the anode 326 of diode D₁ 308 and the voltage at the anode 336 ofdiode D₂ 310. In some implementations, the value of resistor R₁ 302 andresistor R₃ 306 can be selected in the temperature compensating brownoutcircuit 300 such that they are equal to each other. The brownoutreference level, U_(BOR), for the temperature compensating brownoutcircuit 300 can be calculated as follows:

$U_{BOR} = {{k_{1} \cdot \left( {T - T_{0}} \right)} + V_{D0} + {\frac{R_{3}}{R_{2}} \cdot \left( {\frac{k_{B} \cdot T}{q} \cdot {\ln(A)}} \right)}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Temperature, T

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Area Ratio between diodes (chosen by designer), A=10

The selection of A may be based on several factors. For example, if A istoo low the ratio R3/R2 is large, which may cause the circuit to havehigh power consumption and poor accuracy. If A is too large, however,there may be problems with silicon area and leakage currents through thediodes connected in parallel. In some embodiments, the selection of aratio of substantially 10 for A provides a balance between a value thatis too large or too small.

In order for the brownout reference level, U_(BOR), to be independent oftemperature, T, the values of resistor R₃ 306 and resistor R₂ 304 can beselected such that:

${\frac{R_{3}}{R_{2}} \cdot \left( {\frac{k_{B}}{q} \cdot {\ln(A)}} \right)} = {- k_{1}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (chosen by designer), A=10

The brownout reference level, U_(BOR), therefore is:U _(BOR) =V _(D0) −k ₁ T ₀=1.25 volts

where

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

If the power supply voltage V_(CC) 316 is forced lower than the brownoutreference level, U_(BOR), the voltage level at node 322 will be lowerthan the voltage level at node 338. Therefore, the negative input 342 ofthe comparator 312 is forced lower than the positive input 344 of thecomparator 312, causing the output 350 of the comparator 312 totransition indicating a brownout condition. In this example, if thepower supply voltage V_(CC) 316 is below 1.25 volts, a brownoutindication will occur.

FIG. 4 is a schematic of an exemplary temperature compensating brown outcircuit 400 having a variable brownout voltage reference. In the circuitof FIG. 3, the brownout voltage is fixed at approximately 1.25 volts.The temperature compensating brownout circuit 400 includes a voltagedivider 402 coupled to the temperature compensating circuit 110, whichfacilitates changing the brownout voltage reference level. Variation inthe resistor values of the voltage divider 402 permits the temperaturecompensating circuit 110 to specify the brownout reference level.

In this example, the temperature compensating circuit 110 includes thevoltage divider 402, resistors R₁ 404, R₂ 406, and R₃ 408, and diodes D₁410 and D₂ 412. Outputs of the temperature compensating brownout circuit400 are coupled to inputs of a comparator 414.

The voltage divider 402 includes a resistor R₄ 416 and a resistor R₅418. Terminal 420 of resistor R₄ 416 is coupled to a power supplyvoltage V_(CC) 422. Terminal 424 of resistor R₄ 416 is coupled toterminal 426 of resistor R₅ 418 at node 428. Terminal 430 of resistor R₅430 is coupled to a power supply ground 432.

In a first branch of the temperature compensating circuit 110, terminal434 of resistor R₁ 404 is coupled to a node 428 of the voltage divider402. Terminal 436 of resistor R₁ 404 is coupled to terminal 438 ofresistor R₂ 406 at node 440. Terminal 442 of resistor R₂ 406 is coupledto an anode 444 of diode D₁ 410. A cathode 446 of diode D₁ 410 isconnected to the power supply ground 432.

In a second branch of the temperature compensating brownout circuit 400,terminal 448 of resistor R₃ 408 is also coupled to node 428. Terminal450 of resistor R₃ 408 is coupled to an anode 452 of diode D₂ 412 atnode 454. A cathode 456 of diode D₂ 412 is connected to the power supplyground 432.

The voltage divider 402 may be considered a third branch of thetemperature compensating circuit 110, which is coupled to the first andsecond branches at the node 428.

Node 440 is coupled to a negative or inverting input 458 of comparator414. The negative input 458 of comparator 414 is referred to as U_(P′),similarly described in reference to FIG. 1. Node 454 is coupled to apositive or non-inverting input 460 of comparator 414. The positiveinput 460 of comparator 414 is referred to as a brownout referencelevel, U_(BOR), similarly described in reference to FIG. 1. An output462 of comparator 414 is the brownout indicator signal, U_(BO), againsimilarly described in reference to FIG. 1. Comparator 414 is powered bythe power supply voltage V_(CC) 422 at terminal 464 and is grounded atthe power supply ground 432 at terminal 466.

The equations for the current through a diode, I(V_(D),T), and thevoltage across a diode, V_(D)(T), can be applied to the temperaturecompensating brownout circuit 400. The current flowing through resistorR₁ 404 and resistor R₃ 408 can be temperature dependent. Selecting thevalue of resistor R₁ 404 and resistor R₃ 408 equal to each other, thebrownout reference level is found as follows:

$U_{BOR} = {{\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left\lbrack {{k_{B} \cdot \frac{T}{q} \cdot {\ln(A)} \cdot \frac{R_{3}}{R_{2}}} + {k_{1} \cdot \left( {T - T_{0}} \right)} + V_{D0}} \right\rbrack} + {\frac{2}{R_{2}} \cdot k_{B} \cdot \frac{T}{q} \cdot {\ln(A)} \cdot R_{4}}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Temperature, T

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (which may be selected by designer), A=10

The resulting brownout reference level, U_(BOR), is independent oftemperature. Therefore,

${\frac{\mathbb{d}}{\mathbb{d}T}U_{BOR}} = 0$

and after rewriting and collecting the temperature dependent terms

$U_{BOR} = {{\left\lbrack {{\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left( {{\frac{k_{B}}{q} \cdot {\ln(A)} \cdot \frac{R_{3}}{R_{2}}} + k_{1}} \right)} + {\frac{2}{R_{2}} \cdot \frac{k_{B}}{q} \cdot {\ln(A)} \cdot R_{4}}} \right\rbrack \cdot T} + {\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left\lbrack {{\left( {- k_{1}} \right) \cdot T_{0}} + V_{D0}} \right\rbrack}}$

one may derive a temperature insensitive level:

${\frac{\mathbb{d}}{\mathbb{d}T}U_{BOR}} = {0 = {{> {{\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left( {{\frac{k_{B}}{q} \cdot {\ln(A)} \cdot \frac{R_{3}}{R_{2}}} + k_{1}} \right)} + {\frac{2}{R_{2}} \cdot \frac{k_{B}}{q} \cdot {\ln(A)} \cdot R_{4}}}} = 0}}$

Because the temperature dependent terms are set to zero, the brownoutreference level, U_(BOR), can be as follows:

$U_{BOR} = {\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left\lbrack {{\left( {- k_{1}} \right) \cdot T_{0}} + V_{D0}} \right\rbrack}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6˜10⁻¹⁹

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (chosen by designer), A=10

The current through the diodes, D₁ 410 and D₂ 412, can be selected inthe temperature compensating brownout circuit 400 as a design parameter.Therefore, resistor R₂ 406 can be determined by the following equation:

$R_{2} = {\frac{\ln(A)}{I_{2}} \cdot \left( {k_{B} \cdot \frac{T_{0}}{q}} \right)}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Room temperature, T₀=300 K

-   -   Current through the resistor R₂ 406 and the diodes, D₁ 410 and        D₂ 412, when the power supply voltage V_(CC) 422 is at the        brownout reference level, U_(BOR), I₂ Ratio between diodes        (chosen by designer), A=10

After the value of resistor R₂ 406 is determined, the relationshipbetween resistors R₃ 408, R₄ 416, and R₅ 430 can be determined by usingthe equations previously described for the brownout reference level,U_(BOR).

The minimum value resistor R₄ 416 can have is zero. This, in turn,results in a maximum value for resistor R₃ 408 as shown in the followingequation:

$R_{3{MAX}} = {\left( {- k_{1}} \right) \cdot R_{2} \cdot \frac{q}{k_{B} \cdot {\ln(A)}}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (chosen by designer), A=10

In some embodiments, the resistor R₃ 408 is chosen to have a largevalue. However if the value of resistor R₃ 408 is chosen to be toolarge, the value of resistor R₄ 416 can be too small. This can result ina large amount of power consumption for the temperature compensatingbrownout circuit 400. The temperature compensating brownout circuit 400can be designed to have favorable sensitivity to a change in the powersupply voltage V_(CC) 422 at node 440. In one implementation, this isaccomplished by selecting a large value for resistor R₃ 408. Thefollowing equation can characterize this sensitivity:

${Sensitivity} = {\frac{\mathbb{d}}{\mathbb{d}V_{CC}}U_{1}}$

where

U₁ is the voltage level at node 440.

which implies that:

${Sensitivity} = \frac{2 \cdot \left( {{k_{1} \cdot T_{0}} - V_{D0}} \right)}{U_{BOR} \cdot \left( {{\frac{2}{k_{B} \cdot {\ln(A)}} \cdot k_{1} \cdot q} + {\frac{R_{2}}{R_{3} \cdot k_{B} \cdot {\ln(A)}} \cdot k_{1} \cdot q} - 1} \right)}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (chosen by designer), A=10

Once the values of resistors R₂ 406 and R₃ 408 are determined, thevalues for resistors R₄ 416, and R₅ 430 can be determines as shown inthe following equations:

$R_{4} = {U_{BOR} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{2 \cdot \left( {{k_{1} \cdot T_{0}} - V_{D0}} \right) \cdot k_{B} \cdot {\ln(A)}}}$$R_{5} = {\frac{- 1}{2} \cdot U_{BOR} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{k_{B} \cdot {\ln(A)} \cdot \left( {U_{BOR} + {k_{1} \cdot T_{0}} - V_{D0}} \right)}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

Ratio between diodes (chosen by designer), A=10

Brownout reference level, U_(BOR)

For example, the temperature compensating brownout circuit 400 can bedesigned with a brownout reference level, U_(BOR), of substantially 3.5volts. The temperature compensating brownout circuit 400 can be designedwith a current, 12, through the diodes D₁ 410 and D₂ 412 and theresistor R₂ 406 equal to 1 uAmp. The value of resistor R₂ 406 can becalculated to be:

$R_{2} = {{\frac{\ln(A)}{I_{2}} \cdot \left( {k_{B} \cdot \frac{T_{0}}{q}} \right)} = {60\mspace{14mu} k\;{Ohm}}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Room temperature, T₀=300 K

Ratio between diodes (chosen by designer), A=10

I₂=1 uAmp=1×10−6

The maximum value of resistor R₃ 408 can be calculated to be:

$R_{3{MAX}} = {{\left( {- k_{1}} \right) \cdot R_{2} \cdot \frac{q}{k_{B} \cdot {\ln(A)}}} = {600\mspace{14mu} k\;{Ohm}}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Ratio between diodes (chosen by designer), A=10

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K R₂=60kOhm, as determined above

As described above, the value of resistor R₃ 408 can be selected to belarge. This can result in the value of resistor R₄ 416 becoming toosmall, which may cause the circuit to have a large power consumption.Therefore, the value of resistor R₃ 408 may be selected to be less thanits maximum calculated value, 600 kOhm. In one example, R₃ 408 isselected to be 500 kOhm. The value of resistors R₄ 416, and R₅ 430 canbe calculated using the following equations:

$R_{4} = {{U_{BOR} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{2 \cdot \left( {{k_{1} \cdot T_{0}} - V_{D0}} \right) \cdot k_{B} \cdot {\ln(A)}}} = {140\mspace{14mu} k\;{Ohm}}}$$R_{5} = {{\frac{- 1}{2} \cdot U_{BOR} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{k_{B} \cdot {\ln(A)} \cdot \left( {U_{BOR} + {k_{1} \cdot T_{0}} - V_{D0}} \right)}}78{\mspace{11mu}\;}k\;{Ohm}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6˜10⁻¹⁹

Ratio between diodes (chosen by designer), A=10

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

R₂=60 kOhm, as determined above

R₃=500 kOhm, as selected above

Brownout reference level, U_(BOR)=3.5 volts

The sensitivity can then be calculated using the following equation:

${Sensitivity} = {\frac{2 \cdot \left( {{k_{1} \cdot T_{0}} - V_{D0}} \right)}{U_{BOR} \cdot \left( {{\frac{2}{k_{B} \cdot {\ln(A)}} \cdot k_{1} \cdot q} + {\frac{R_{2}}{R_{3} \cdot k_{B} \cdot {\ln(A)}} \cdot k_{1} \cdot q} - 1} \right)} = {32\mspace{14mu}{mV}\text{/}V}}$

where

Boltzmann's constant, k_(B)=1.38·10⁻²³

Electron voltage, q=1.6·10⁻¹⁹

Ratio between diodes (chosen by designer), A=10

Room temperature, T₀=300 K

Diode voltage at room temperature (process dependent), V_(D0)=0.65 V

Diode voltage temperature gradient (process dependent), k₁=−2 mV/K

R₂=60 kOhm, as determined above

R₃=500 kOhm, as selected above

Brownout reference level, U_(BOR)=3.5 volts

Therefore, if the power supply voltage V_(CC) 422 changes by 1 volt, thevoltage level at node 440, which is coupled to the negative input 458 ofcomparator 414, will change by 32 mV. The sensitivity can be used toselect the design constraints for the comparator 414.

The brown out device may be included in a system, such as the system 500shown in FIG. 5. The system 500 includes a processor 510, a memory 520,a storage device 530, and one or more input/output devices 540. Each ofthe components 510, 520, 530, and 540 can be interconnected using asystem bus 550. In some implementations, the processor 510 is capable ofprocessing instructions for execution within the system 500. Forexample, the processor 510 can be the processing unit 108 that executesinstructions that carry out the optional step 212 of the method 200. Thebrownout detector 106 may be integrated into the system 500 so that itmonitors the power supply voltage provided to the processor 510, thememory 520, the storage device 530 or the I/O devices 540.

In some implementations, the processor 510 is a single-threadedprocessor. In other implementations, the processor 510 is amulti-threaded processor. The processor 510 is capable of processinginstructions stored in the memory 520, or on the storage device 530. Insome implementations, the processed instructions may generate graphicalinformation for a user interface, on one of the input/output devices540.

The memory 520 stores information within the system 500. In someimplementations, the memory 520 is a computer-readable medium. In someimplementations, the memory 520 is a volatile memory unit. In otherimplementations, the memory 520 is a non-volatile memory unit.

The storage device 530, such as the memory 106, is capable of providingmass storage for the system 100. In some implementations, the storagedevice 530 is a computer-readable medium. In various differentimplementations, the storage device 530 may be a floppy disk device, ahard disk device, an optical disk device, or a tape device.

The input/output devices 540 provide input/output operations for thesystem 500. In some implementations, the input/output devices 540include a keyboard and/or pointing device. In other implementations, theinput/output devices 540 include a display unit for displaying graphicaluser interfaces.

The features described can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The apparatus can be implemented in a computerprogram product tangibly embodied in an information carrier, e.g., in amachine-readable storage device or in a propagated signal, for executionby a programmable processor; and method steps can be performed by aprogrammable processor executing a program of instructions to performfunctions of the described implementations by operating on input dataand generating output. The described features can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. A computer program is a set of instructionsthat can be used, directly or indirectly, in a computer to perform acertain activity or bring about a certain result. A computer program canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) monitor for displaying information tothe user and a keyboard and a pointing device such as a mouse or atrackball by which the user can provide input to the computer.

The components of the system can be connected by any form or medium ofdigital data communication such as a communication network. Examples ofcommunication networks include, e.g., a LAN, a WAN, and the computersand networks forming the Internet.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the subject matter. For example, the comparator112, described in reference to FIG. 1, can determine when the dividedpower supply voltage level rises above the temperature compensatedvoltage reference. When this occurs, the output 116 of the comparator112 will transition from a first voltage level, for example zero volts,to a second voltage level, for example 2.5 volts, indicating theapplication of power to the computing device 104. The computing device104 can use this indicator to power up and reset its components. In yetother implementations, the power supply voltage is not divided before itis input into the comparator. Additionally, the grounds 330 and 432 ofFIGS. 3 and 4, respectively can be floating grounds instead of powersupply grounds. Accordingly, other embodiments are within the scope ofthe following claims.

1. A brownout detection circuit comprising: a comparator to detect acrossing of a brownout threshold that is specified by a referencevoltage, wherein the comparator has a first and a second input; and atemperature compensating circuit having two branches connected inparallel, a first branch to provide the reference voltage substantiallyindependent of temperature and a second branch to provide an indicationof a supply voltage, wherein the first branch is coupled to the firstinput and the second branch is coupled to the second input.
 2. Thedetection circuit of claim 1, wherein the temperature compensatingcircuit further comprises a third branch having components to adjust thereference voltage.
 3. The detection circuit of claim 2, wherein thecomponents include two resistors in series.
 4. The detection circuit ofclaim 3, wherein the third branch is coupled to the first and secondbranch at a node between the resistors.
 5. The detection circuit ofclaim 1, wherein the first branch comprises a first resistor and asecond resistor in series.
 6. The detection circuit of claim 5, whereinthe first branch is coupled to the first input at a first node betweenthe first and second resistors.
 7. The detection circuit of claim 5,wherein the first branch further comprises a first diode coupled inseries with the second resistor and having its cathode grounded.
 8. Thedetection circuit of claim 1, wherein the first input of the comparatoris a negative input or an inverting input.
 9. The detection circuit ofclaim 1, wherein the second branch comprises a third resistor.
 10. Thedetection circuit of claim 9, wherein the second branch furthercomprises a second diode in series with the third resistor and havingits cathode grounded.
 11. The detection circuit of claim 10, wherein thesecond branch is coupled to the second input at a second node betweenthe third resistor and the second diode.
 12. The detection circuit ofclaim 1, wherein the second input of the comparator is a positive inputor a non-inverting input.
 13. The detection circuit of claim 1, whereinthe first branch comprises a first resistor, a second resistor, and afirst diode coupled in series, and the second branch comprises a thirdresistor and a second diode coupled in series, wherein the first andsecond branches are coupled at a node between the first resistor and thethird resistor.
 14. The detection circuit of claim 13, furthercomprising a third branch coupled to the first and second branches, thethird branch having a fourth resistor, a fifth resistor, and a nodebetween the fourth and fifth resisters that is coupled to the nodebetween the first resistor and the third resistor.
 15. The detectioncircuit of claim 14, wherein a relationship between the resistors andthe diodes is expressed by${{{\left( {\frac{R_{4}}{R_{5}} + 1} \right) \cdot \left( {{\frac{k_{B}}{q} \cdot {\ln(A)} \cdot \frac{R_{3}}{R_{2}}} + k_{1}} \right)} + {\frac{2}{R_{2}} \cdot \frac{k_{B}}{q} \cdot {\ln(A)} \cdot R_{4}}} = 0},$where k_(B) is Boltzmann's constant, q is an electron charge constant, Ais a ratio between the diodes, and k₁ is a voltage temperature gradientof the diodes.
 16. The detection circuit of claim 15, wherein the secondresistor has a resistance value of${R_{2} = {\frac{\ln(A)}{I_{2}} \cdot \left( {k_{B} \cdot \frac{T_{0}}{q}} \right)}},$where I₂ is a value of a current passing through the second resistor,and To is a temperature of an environment surrounding the temperaturecompensating circuit.
 17. The detection circuit of claim 16, wherein thethird resistor has a maximum resistance value of$R_{3{MAX}} = {\left( {- k_{1}} \right) \cdot R_{2} \cdot {\frac{q}{k_{B} \cdot {\ln(A)}}.}}$18. The detection circuit of claim 17, wherein the fourth resistor has aresistance value of${R_{4} = {V_{REF} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{2 \cdot \left( {{k_{1} \cdot T_{0}} - V_{D0}} \right) \cdot k_{B} \cdot {\ln(A)}}}},$where V_(REF) is the voltage reference, T₀ is a temperature of anenvironment surrounding the temperature compensating circuit, and V_(D0)is a voltage of the diodes at the temperature of the environment. 19.The detection circuit of claim 17, wherein the fifth resistor has aresistance value${R_{5} = {\frac{- 1}{2} \cdot \frac{{k_{B} \cdot {\ln(A)} \cdot R_{3}} + {k_{1} \cdot q \cdot R_{2}}}{k_{B} \cdot {\ln(A)} \cdot \left( {V_{REF} + {k_{1} \cdot T_{0}} - V_{D0}} \right)}}},$where V_(REF) is the voltage reference, T₀ is a temperature of anenvironment surrounding the temperature compensating circuit, and V_(D0)is a voltage of the diodes at the temperature of the environment.
 20. Amethod for brownout detection comprising: setting a brownout thresholdvoltage using a first branch of a temperature compensating circuit,wherein the brownout threshold voltage is substantially independent oftemperature changes; providing a second voltage using a second branch ofthe temperature compensating circuit in parallel with the first branch,wherein the second voltage indicates a power supply voltage; andgenerating a signal from a comparator when the second voltage crossesthe brownout threshold voltage, wherein the brownout threshold voltageis coupled to a first input of the comparator and the second voltage iscoupled to a second input of the comparator.
 21. The method of claim 20,further comprising selecting the brownout threshold voltage.
 22. Acircuit comprising: a detection component to detect a crossing of areference voltage by an input voltage; and a temperature compensatingcircuit to generate the reference voltage, the temperature compensatingcircuit comprising two circuit branches connected in parallel andseparately coupled to the detection component, a first circuit branch togenerate the reference voltage being substantially independent oftemperature, and a second circuit branch to generate the input voltage.23. A method for brownout detection comprising: setting a brownoutthreshold voltage, maintaining the brownout threshold voltagesubstantially independent of temperature changes including compensatingtemperature changes in parallel circuit elements to substantiallymaintain the brownout threshold voltage at a substantially constantlevel; evaluating a second voltage indicative of a power supply voltage;and generating a signal when the second voltage crosses the brownoutthreshold voltage.